The present invention relates generally to electronic article security (EAS) systems and more particularly to an improved numerically controlled oscillator for controlling the operating frequency of an EAS system.
In general, EAS systems are used for detecting and preventing theft or unauthorized removal of articles which are readily accessible to potential customers or facility users and are susceptible to unauthorized removal. Such EAS systems generally employ a security tag which is secured to or associated with an article or it's packaging. The EAS systems detect the presence (or absence) of the security tag, and thus the presence or absence of a protected article, within a detection zone. Typically, the detection zone is located at or around an exit or entrance to the facility or a portion of the facility.
One type of EAS system which has gained widespread popularity utilizes a security tag which includes a self-contained resonant circuit in the form of a small, generally planar printed circuit which resonates at a predetermined detection frequency within a detection frequency range. A transmitter which is tuned to the detection frequency is employed for transmitting electromagnetic energy into the detection zone. A receiver, also tuned to the detection frequency, is positioned proximate to the detection zone. When an article having an attached security tag passes into or through the detection zone, the security tag is exposed to the transmitted electromagnetic energy resulting in the resonant circuit resonating to provide an output signal detectable by the receiver. The detection of such an output signal by the receiver indicates the presence of an article with an attached security tag within the detection zone and the receiver actuates an alarm to alert appropriate security or other personnel.
EAS systems of the type described above employ a transmitter to provide a radio frequency (RF) output signal to a transmit antenna. In one kind of generally employed EAS system the frequency of the output signal is swept up and down at a predetermined sweep rate within a predetermined frequency range generally surrounding the resonant frequency of the tags employed. Typically, the output frequency is swept between a low frequency of 7.2 MHz. and a high frequency of 9.2 MHz. and thus has a bandwidth of 2.0 MHz. and a center frequency of 8.2 MHz. Security tags typically employed with the EAS system have a resonant frequency of 8.2 MHz. but may vary upwardly or downwardly due to a variety of factors including manufacturing tolerance, environmental conditions, etc. By sweeping through a band on both sides of the tag nominal resonant frequency, the EAS system compensates for such tag variations and is able to reliably detect a high percentage of all security tags.
In use, the EAS system transmitter emits the swept frequency into the detection zone from the transmit antenna. The emitted RF signal is received by a receive antenna and is demodulated by the EAS receiver. Where no security tag is present in the detection zone, the receiver detects a known pattern. The presence of a resonant security tag in the detection zone causes the received pattern to deviate from the known pattern in recognized ways resulting in the generation of an alarm as described above.
Security tags are made in high volume and require rapid individual testing to ensure that they will respond properly to EAS systems when attached to a protected article. Security tags having a resonant frequency outside predetermined limits or having a resonance with insufficient Q are normally rejected by the testing process. In this case, quantitative measurements of the security tag resonant frequency and indications representative of the tag Q are required to be performed at high speed. EAS systems adapted to testing are preferable for performing the tag measurements because the tag characteristics can be measured without contacting the individual tags.
Current EAS system transmitters typically use voltage controlled oscillators (VCOs) employing varactor diodes as variable capacitor elements to enable the frequency of the voltage controlled oscillator to be swept between the low and high limits. The nature of varactor diodes results in instability of the frequency of the voltage controlled oscillator output signal and also results in a non-linear frequency sweep characteristic. From a security tag testing perspective, the frequency instability of the transmitted signal adds uncertainty in measuring the resonant frequency of the tag being tested. From an EAS system operating perspective, VCO instability requires the EAS transmitter to sweep over an even larger bandwidth to compensate for the VCO instability or alternatively, forces narrower production limits on the tag resonant frequency. In the former case, the frequency instability of the transmitted signal reduces the reliability of tag detection since the acceptance limits of the received signal must be made larger. In the latter case, narrower production limits on tag resonant frequency increases the tag reject rate and thus costs. Also, the non-linear sweep characteristic of the frequency sweep has undesired effects, principally in reducing the probability of detection, increasing the false alarm rate and increasing the out-of-band emissions.
The availability of high volume large scale integrated (LSI) circuits has made it economically feasible to employ direct digital synthesis devices in EAS systems in place of varactor tuned voltage controlled oscillators. The direct digital synthesis device, when controlled from a high stability clock such as a crystal oscillator, substantially eliminates frequency drift and the attendant detection losses due to frequency drift. In addition, the use of a direct digital synthesis device in EAS systems allows for the generation of a wide variety of accurately controlled frequency patterns, which could include arbitrary frequency patterns such as pseudo random patterns in addition to the linear and sinusoidal frequency sweep patterns typically used in EAS systems, with potential improvement in the probability of detection, reduced false alarm rate and reduced out-of-band emissions.